DTIC ADA481572: Power Module Cooling for Future Electric Vehicle Applications: A Coolant Comparison of Oil and PGW

POWER MODULE COOLING
FOR FUTURE ELECTRIC VEHICLE APPLICATIONS:
A COOLANT COMPARISON OF OIL AND PGW

T. E. Salem
U. S. Naval Academy
105 Maryland Avenue
Annapolis, MD 21402

D. P. Urciuoli

U. S. Army Research Laboratory
2800 Powder Mill Road
Adelphi, MD 20783

ABSTRACT

Compact and efficient power converters are being
developed to support the needs of future ground vehicle
systems. This progress is being driven by component
advancements, combined with improvements in
component thermal management achieved through
various liquid cooling implementations. Regardless of the
configuration of the thermal management system, the
properties of the liquid coolant used are vital to its
performance. This work compares the use of turbine oil
and an aqueous glycol solution as coolants in an
automotive based power converter application.

1. INTRODUCTION

Advancements in power electronic conversion system
technologies will enable next generation ground vehicles
to fulfill increasingly demanding mission objectives. DC-
DC converter and inverter systems slated for future
propulsion, survivability, and lethality applications,
operate at power levels on the order of 100 kW and
above. Even with very high efficiencies, the components
of these systems produce kilowatts of power loss in the
form of heat. Benefits can be realized by improving
thermal management of active and passive system
components. However, more effective cooling of active
electronic devices can also enable higher output power at
the converter’s system level.

The most widely used implementation for thermal
management of active electronic components is an air
cooled heat sink approach. In its simplest and most
primitive form, air cooling is achieved passively through
natural convection. Improved heat dissipation can be
realized by the addition of forced air flow channeled in
both laminar and impinging directions. However, as
system volumetric power densities increase, inherent
material properties preclude an air cooled approach.
Other cooling methods, such as thermoelectric and active

spray cooling have been demonstrated with promising
results. However, widespread implementation of these
techniques has not yet occurred.

Presently, liquid cooling is the most viable approach
to meet system design parameters and has been used in a
variety of industrial and military applications. In vehicle
systems, automotive fluids such as engine oil and engine
coolant are readily available for electronic cooling
applications. Future military vehicle system design
requirements have varied between using one of these two
fluids. However, neither fluid is optimized for power
component heat exchange in composition or operating
temperature. Factors such as electrical conductivity,
density, viscosity, specific heat, and thermal conductivity,
can make one fluid more suitable than the other. This
paper presents performance and material property data for
Castrol 399 turbine oil and 50% by weight aqueous
solutions of ethylene (WEG) and propylene (PGW)
glycol. PGW is replacing WEG in most automotive
applications because it offers nearly identical properties
without the toxic environmental effects. Results of
experiments conducted using both Castrol 399 and PGW
as cooling fluids are shown. From these results,
conclusions are drawn regarding their use in cooling
system designs.

2. FLUID PROPERTIES

Using fluid as a coolant in an electrical system
appears counterintuitive because many fluids are
electrically conductive. Of the two types of automotive
fluids considered, aqueous glycol solutions are
electrically conductive, while engine oils are not. As a
result, oil has the advantage of being used in direct
contact with electrically active heat generating surfaces.
By contrast, aqueous glycol solutions require the active
surface to have an electrical isolation layer, thereby
increasing the thermal resistance of the interface.

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Power Module Cooling For Future Electric Vehicle Applications: A
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Performance of liquid cooled heat exchangers is
impacted by the fluid flow rate, which is directly related
to fluid density and viscosity. Castrol 399 and WEG have
comparable fluid densities which do not vary significantly
with temperature [1,2]. However, the viscosity of both
coolants does greatly vary over the temperature range.
Castrol 399 has a viscosity that is approximately five
times higher than that of WEG for a given cooling system
operating temperature between 25° C and 80° C. The
change in oil viscosity over this range is also nearly twice
as large as that of WEG, which negatively impacts the
overall system pump requirements at low temperatures
[1,2].

Two important properties affecting the thermal
transfer capability for fluids are specific heat and thermal
conductivity. Specific heat is the measure of the
temperature rise of a given volume of material as a
function of its absorption of thermal energy. At 50° C,
Castrol 399 has a specific heat of 2.0 (kJ-kg'^K"1),
compared to 3.5 (kJ-kg^K1) for WEG [1,2]. Therefore, a
given volume of Castrol 399 will exhibit nearly twice the
temperature rise as that of WEG for the same amount of
absorbed thermal energy. Thermal conductivity is a
metric of the ability of a material to internally transfer
thermal energy. Between 25° C and 80° C, the thermal
conductivities of Castrol 399 and WEG are approximately
0.15 (W-m'-K1) and 0.40 (W-m'-K1), respectively [1,2],
For a given volume, geometry, and thermal power input
for each fluid, the maximum temperature of Castrol 399
can be nearly three times that of WEG. Table 1 displays
the values of the properties discussed for Castrol 399 and
WEG.

Table 1. Fluid Property Values (40° C)

Fluid

Castrol 399

WEG

Density (kg-m'3)

937

1058

Viscosity (mPa-s)

15.3

2.3

Specific Heat (kJ-kg '-K1)

2.0*

3.5*

Thermal Conductivity (W-nC-IC1)

0.15

0.40*

periods of high power demand, such as vehicle
acceleration, high torque conditions, or loading from
auxiliary subsystems, the BDC operates in boost-mode by
stepping-up the battery voltage and providing additional
power to the propulsion bus. Conversely, under light
propulsion loads, the BDC operates in buck-mode,
stepping-down the high voltage from the propulsion bus
and recharging the battery bank [3]. Fig. 1 shows the
ARL 90 kW BDC test bed platform. The circuit design of
the 90 kW BDC test platform consists of three-phases
each using a common half-bridge IGBT switch module
with incorporated anti-parallel diodes. This type of
module is readily available due to its widespread use in
both DC and AC power conversion systems.

Fig. 1. ARL 90 kW bi-directional converter platform.

For this study, the test platform was modified to
operate as a single-phase boost converter. Fig. 2 shows
the corresponding circuit diagram with Qi and Q2
representing the half-bridge IGBT switch module [3].
The diagram shows the low voltage battery (LS) and high
voltage propulsion bus (HS). This implementation
facilitated acquiring thermal imaging data of the IGBT die
surfaces. Additionally, this configuration ensures a
known power flow through the individual module tested
without compromising converter function.

* Data values reported at 50° C.

3. EVALUATION PLATFORM

A major part of the Army Research Laboratory’s
(ARL) hybrid electric vehicle research and development
program, funded through TARDEC, is the design and
fabrication of a high power bi-directional DC-DC
converter (BDC). This converter manages power flow
between the lower voltage battery pack and the higher
voltage propulsion power bus. Under conventional
operating conditions, propulsion power is provided by a
generator, driven by a diesel engine. However, during

To evaluate the performance of Castrol 399 and
PGW, the IGBT switch module was mounted on a D6
Industries liquid cold plate (Hydroblok-Al-4P-06).
Castrol 399 was pumped through the cold plate using a
Mydax heat exchanger to regulate flow rate and inlet oil
temperature. After internally cleaning the oil from the
cold plate, PGW testing was conducted using a Julabo
heat exchanger. Arctic Silver® thermal compound was
applied to the cold plate and component interface. The
module used in the evaluation was a commercially
available Powerex CM400DU-24NFH dual 400 A, 1200 V
IGBT half-bridge module. This part contains IGBT die
optimized for fast switching applications. Compared to
other modules, the switching losses are lower while the
conduction losses are higher. The module case was
opened and the protective potting compound was
removed to expose the die surfaces for thermal imaging,
as shown in Fig. 3. For accurate infrared (IR) thermal
measurement, the die surfaces were uniformly coated with
boron nitride [4]. IR imaging of the active IGBT die
surfaces was achieved using a FLIR ThermaCAM SC500.

Fig. 3. Exposed IGBT half-bridge module.

During testing, the converter was operated using
open loop control at various output power levels ranging
from 5 kW to 30 kW with each cooling fluid. The input
voltage level was 300 V with an output load voltage of
600 V and the switching frequency was 17 kHz. Based
on the inductor value, the converter operated in
discontinuous conduction mode during all tests. This
mode of operation minimizes the turn-on switching loss
of the IGBT and decreases the diode loss.

4. EXPERIMENTAL RESULTS

The experimental test setup was configured for the
Castrol 399 coolant at a heat exchanger set point of 25° C.
A resistive load bank was used to step the converter
output power through the operating range. Each power
level test point was maintained for five minutes to attain
thermal equilibrium of the IGBT die. This test procedure

was repeated with the temperature set point of the Castrol
399 raised to 50° C. The cooling loop was then
configured for PGW and tests were conducted at both 25°
C and 50° C. Table 2 summarizes the flow rates of the
fluids during the tests. For each power level test point,
the active die surface temperature was measured with the
IR camera and a thermal image was captured. Fig. 4
shows thermal images of the device for each cooling fluid
at 25° C and 25 kW output. The left image shows the
thermal effects of PGW cooling while the right image
shows the effects of Castrol 399 cooling. The spectrum
of these images reveals that the oil cooled device had a
significantly higher operating temperature (114° C) than
the PGW cooled device (78° C).

Table 2. Coolant Flow Rates for Tests

Fluid

Castrol 399

PGW

Temperature (° C)

25

50

25

50

Flow Rate (gpm)

1.51

1.61

1.55

1.37

i ■ ■ * ■ i ■ ■ i ■ ' ■ ■ i ■ ■ ■ ' j ■ ' i ■ 1 1 ■ ■ ■ i ' ■ ■ 1 1 ■ ' 1 1 1 1 j ■ ■ | j ■ 1 1 ) ■ 1 1 ■ 1 1 1 ■ ■ |

60 aC 70 B0 90 100 110 120X

Fig. 4. Die thermal images at 25 kW output, 25° C
Coolant (PGW left, Castrol 399 right).

Fig. 5 shows a graph of test data relating maximum
die temperature to the converter output power level for
both Castrol 399 and PGW at 25° C. Similarly, Fig. 6
presents 50° C coolant test data. To provide sufficient
safety margin for protecting the IR camera from device
failure, maximum operating temperature of the IGBT die
was limited to 120° C. The 27 kW test data of Fig. 5
shows that the IGBT temperature reached 120° C when
cooled with Castrol 399, compared to only 84° C when
cooled with PGW. Likewise, the 20 kW test data of Fig.
6 shows that the IGBT temperature reached 116° C when
cooled with Castrol 399, compared to only 92° C when
using PGW. The trend of the data reveals that when using
Castrol 399, over a 3° C rise in die surface temperature
occurs for each kilowatt increase in converter output
power. Similarly, PGW exhibits a 2° C rise in die surface
temperature for each kilowatt increase in converter output
power.

Fig. 5. Fluid comparison at 25° C coolant temperature.

Fig. 6. Fluid comparison at 50° C coolant temperature.

5. CONCLUSION

The combination of several factors makes one
cooling fluid more favorable than another for use as a
liquid coolant in electrical systems. Onboard a vehicle
having stringent space and weight limitations, using an
already available vehicle fluid for cooling electronic
systems is recommended. Both engine oil and aqueous
glycol solutions have been proposed as coolants for
vehicle power converters. However, several physical
factors make aqueous glycol solutions the better
performing and more favorable choice of coolant over
engine oil. Despite the desirable electrical insulating
property of oil, many of its other properties contribute to
its poor performance as a cooling fluid.

The expression of temperature rise as a function of
power is commonly known as thermal resistance.
Achieving high volumetric power density in electronic
systems requires an optimization of thermal performance,
which means that the system thermal resistance must be
minimized. Using Castrol 399 oil as coolant yielded a
50% increase in overall thermal resistance compared to
the same system using PGW as coolant. This significant
result can be viewed from two vantage points. First, for a
given maximum operating temperature of a power
converter, using an aqueous glycol solution instead of
engine oil as coolant enabled system power to be
increased by 50%. Second, for a power converter
operating at a specific load point, the temperature rise of
the system when cooled with an aqueous glycol solution
can be half as much as the temperature rise of the same
system when cooled with an engine oil.

REFERENCES

[1] T. Sun and A. S. Teja, “Density, Viscosity, and

Thermal Conductivity of Aqueous Ethylene,
Diethylene, and Triethylene Glycol Mixtures
Between 290 K and 450 K,” J. Chem. Eng.
Data., vol. 48, pp. 198-202, 1990.

[2] “MIL-L-7808 Properties.” Data Book for Designers,

(communication with British Petroleum).

[3] D. Urciuoli and C. W. Tipton, “Development of a 90

kW Bi-Directional DC-DC Converter for Power
Dense Applications,” 21st Annual IEEE Applied
Power Electronics Conference and Exposition,
2006, pp. 1375-1378.

[4] T. E. Salem, D. Ibitayo, and B. R. Geil, “Calibration

of an Infrared Camera for Thermal
Characterization of High Voltage Power
Electronic Components”, Proceedings of the
IEEE Instrumentation and Measurement
Technology Conference, 2005, Vol. 2, pp. 829-
833.